Butterfly valves, in general, are well known and widely employed due to their simplicity of construction and relatively inexpensive cost, as compared to more complicated and detailed plug and ball valves. A typical butterfly valve generally comprises a disc mounted for rotation between the open position, in which the disc lies substantially parallel to the axis of the fluid flow channel through the valve, and the closed position, in which the disc lies perpendicularly to this axis. The disc is mounted for rotation on a valve stem or shaft, which is attached to the disc on one side. The disc cooperates with an annular flexible seat circumscribing the fluid flow channel for the purpose of effecting a resilient seal against the disc to shut off fluid flow through the channel. The annular flexible seat is conventionally held in position by being clamped in a recess formed between complementary surfaces of a portion of the valve body and a valve seat retainer.
During rotation of the disc between the open position and the closed position, the peripheral edges of the disc, particularly those nearest the axis about which the disc rotates, remain in contact with the flexible seat and experience considerable friction during rotation of the disc, causing wear on both the seat and the peripheral edges of the disk. Such wear can lead to the disc failing to seal properly against the valve seat when the disc is in the closed position. Repairing or replacing the valve seat or the disc can be time consuming and expensive. Additionally, an actuator driving the disc wastes power overcoming the sliding friction force between the disc and the valve seat.